Inkjet printers have found widespread use in businesses and homes, in part due to their relatively low cost, high print quality and colour printing capability. Inkjet printers operate by depositing drops of one or more inks onto a print medium such that in combination the drops form the desired image. Deposition of the ink drops is governed by one or more printheads, which are each connected to an ink supply and contain an arrangement of nozzles and a mechanism which is adapted to control ejection of ink drops from each individual nozzle. Typically, each printhead may comprise an array of addressable piezoelectric and/or heating elements corresponding to each nozzle. Each element can be individually charged in order to superheat a small quantity of ink which is subsequently ejected via the corresponding nozzle. The one or more printheads may be housed in a print cartridge (or alternatively each printhead may be housed in a separate cartridge), which may either contain the supply of ink for each printhead (for so-called ‘on-axis’ printers) or be connected to an ink supply located externally of the cartridge (for so-called ‘off-axis printers). Typically, an inkjet printer will comprise a number of different ink colours which are used in combination to produce a wide range of image colours on the print medium. An inkjet printer may include four print cartridges but may include more or fewer as is appropriate for the intended application. Typically, the print heads are mounted side-by-side in a carriage which scans in a forward and a backward direction above and relative to the print medium during printing such that the cartridges move sequentially over a given location on the medium which is to be printed. Upon each scan, the one or more printheads can address a width of the print medium corresponding to the layout of the nozzles on each cartridge, thus forming a swath. The printer also has a print medium advance mechanism which moves the medium relative to the printhead so that, by combining successive scans of the print cartridges back and forth across the medium with the advance of the medium relative to the printhead, the entire printable area of the medium can be printed using a plurality of swaths.
Each printhead deposits ink of a different colour on the medium. In one commonly used arrangement, the inks are the subtractive colours cyan (C), magenta (M) and yellow (Y). Alternatively the printer can use more than three colour inks, some of which are lighter and darker versions of a given colour shade. Often, there is also a black ink (K), drops of which may be used during colour printing to form the darker shades of colours. Drops of the colour inks can be combined at substantially the same location to form a range of perceived colours to the human eye. For example, superimposing drops of magenta and cyan in the same pixel location produces a purple colour pixel.
In digital form, a colour image may be encoded in a particular colour space, the most common of which is the RGB colour space. Prior to printing the image it much first be converted from its native input colour space to the device colour space appropriate for the printer. For example, an image encoded in the RGB colour space, which is to be printed using a printer with cyan, magenta, yellow and black inks (CMYK), must first be converted to the CMYK colour space.
Typically, the resolution at which an image is to be printed is measured in pixels per inch (ppi). Conversely, the resolution at which a printhead can deposit ink on the print medium is typically measured in dots per inch (dpi) and is governed by the geometry of the printhead (namely the nozzle arrangement). Generally, the dpi value is several times the ppi value in order to ensure accurate representation of the image. For example, an image at 600 ppi may be printed using a printer at a resolution of 2400×1200 dpi, where each image pixel corresponds to a 4 by 2 array of device pixels.
In order to increase the number of colour shades that can be printed, it is common to use halftone techniques for each of the subtractive colours. Halftone techniques use a pattern of dots in combination to simulate a continuous tone of an image as perceived by the human eye at macroscopic level. Thus, the colour of each image pixel is simulated at the dot level using a halftone image for each ink. Generation of each halftone image may be performed using software or hardware implemented methods, as is established in the art.
Some printers operate in a mode in which they only deposit ink while scanning in one direction; for example, in the forward pass but not the backward pass. This print mode is known as unidirectional printing. Conversely, some printers have a mode in which they deposit ink while scanning in both directions, known as bidirectional printing. Bidirectional printing has the advantage of reduced printing time, because it avoids the wasted motion of moving the carriage in the backward direction without printing required for unidirectional printing.
The gamut of a printer refers to the range of colours which can be produced by the device. In order to accurately reproduce all colours contained in an image, it is desirable to maximise the gamut of the printing device. Accordingly, a need exists for a methods and apparatus that can maximise the achievable gamut of printing devices.